Metallic coating films sparkle when metallic flake pigments contained therein reflect incident light. The reflection, in combination with the colors of the coating films, produces an attractive, unique appearance full of variations. The metallic coating films are formed chiefly on exterior panels of automobiles, motorcycles, etc.
Conventional techniques for forming the metallic coating films include the 1-coat 1-bake technique comprising the steps of applying a metallic topcoat composition containing a metallic pigment on a substrate optionally coated with a primer or with a primer and intercoat, and heat-curing the metallic topcoat; the 2-coat 2-bake technique comprising the steps of applying and heat-curing a metallic coating composition, and applying and heat-curing a clear coating composition; the 2-coat 1-bake technique comprising the steps of applying a metallic coating composition and a clear coating composition in this order and heat-curing the two coats at the same time; and the 3-coat 2-bake techniques comprising the steps of applying a clear coating composition to the coating surface formed by the 2-coat 1-bake technique, followed by heat curing.
A metallic coating film having uniform metallic appearance without metallic mottling can be obtained by orientating the metallic flake pigment parallel to the coating surface and uniformly and regularly over the whole coated area, and by providing a metallic coating surface with excellent smoothness.
For satisfying these requirements, it is preferable to use a metallic coating composition which shows a high viscosity when applied to the surface of the substrate and whose film thickness after heat curing is significantly different from the film thickness immediately after application. That is, if the metallic coating composition shows a high viscosity when applied, the metallic pigment is difficult to flow and thus is prevented from causing metallic mottling, and if the film thickness after heat curing is greatly different from the film thickness immediately after application, the metallic pigment is readily orientated parallel to the coating surface.
Accordingly, low solids coating compositions containing a large proportion of organic solvent have been chiefly utilized as metallic coating compositions. In recent years, however, use of aqueous metallic coating compositions is strongly required from the viewpoints of prevention of environmental pollution and conservation of resource.
Although aqueous metallic coating compositions are not likely to cause problems such as environmental pollution, their finish appearances are not fully satisfactory since the evaporation amount of water as the medium changes according to variation of humidity at the time of application, altering the orientation of the metallic pigment.
Recently proposed aqueous metallic coating compositions include one comprising a phosphoric ester group- and alkylene oxide group-containing copolymer and a metallic pigment (Japanese Unexamined Patent Publication No. 25578/1992); and one comprising a phosphoric ester group- and hydroxyl group-containing copolymer and a metallic pigment (Japanese Unexamined Patent Publication No. 271580/1993). The publications discloses that these coating compositions prevent corrosion of metallic pigments.
The present inventors' research, however, found that the proposed aqueous metallic coating compositions have the defects of insufficient orientation of the metallic pigment, insufficient smoothness of the coating film, and unsatisfactory finish appearance.